The present invention relates to a drawing apparatus in a press, with a pressure plate or pressure member which is adapted to be lifted and lowered by way of draw cylinders adapted to be acted upon by means of elastically operating pressure media for the counter-holding during the drawing operation and for the ejection of the sheet metal draw part and which includes at least one blocking device for the delayed trailing of the pressure plate to the upward movement of the press ram.
In order to avoid the simultaneous lifting of the pressure plate with the upward movement of the press ram and therewith an immediate lifting of the sheet metal draw part out of the bottom tool, the pressure plate is retained initially in the lowered position by way of a blocking cylinder until the ram has traversed a return stroke of, for example, more than the drawing depth.
It is known from the DE-PS No. 656 684 to connect the blocking cylinder with the drawing apparatus in that the piston rod of the blocking cylinder is simultaneously piston rod of the draw cushion. It is known from the DE-AS No. 24 08 096 to permit the piston rod, of the draw cushion, and the piston rod of a blocking cylinder to act separately on the pressure plate. The valve-controlled flow of hydraulic liquid between the pressure spaces above and below the pressure piston in the blocking cylinder during the drawing and the ejection of the sheet metal drawing part is common to both drawing apparatus. The hydraulic liquid is thereby heated up to a not-insignificant extent. The flow characteristics of the hydraulic liquid change. The heat must be removed under additional energy losses. The flow of the hydraulic liquid in the blocking device reduces the velocity of the draw cushion during the upward movement.
In contrast thereto, it is the object of the present invention to cause the blocking device to be selectively rendered operable or inoperable. A continuous flow of hydraulic liquid during the periods of time of the inoperative condition is thereby to be precluded.
The underlying problems are solved according to the present invention in that the pressure piston is displaceably supported on the piston rod pivotally connected with the pressure plate, in that the piston rod protrudes out of the side opposite the operating or active surface of the pressure piston and includes thereat an abutment, and in that the side of the pressure piston opposite the operating or active surface is constantly acted upon with pressure so that the pressure piston will move away from the abutment when the operating or active surface of the pressure piston is not acted upon with a pressure, and the pressure piston is placed against the abutment when the operating or active surface of the pressure piston is acted upon with the pressure of a hydraulic liquid.